Twitter has been focusing on content a lot more recently, it's somewhat of a trend for companies that used to focus on just doing one thing well.
These days, Twitter is more of a media company than an information network, so it makes sense that media would get better treatment on the site.
One thing you'll probably notice straight away is how Twitter works with photos now. Photos have been an important part of the site for quite a while, but it's now easier to just view photos from a tweet without disrupting what you were doing.
Basically, Twitter has a lightbox now, just like Facebook, Google+ and many others. You click on a photo, or a video for that matter, and it pops up in place, on top of the page you're viewing. When you're done watching, simply close the overlay.
Speaking of video, the media galleries visible on all profiles, now include videos from YouTube, Vimeo, Vine and everyone else, alongside the photos that have been there until now.